Expanded slags made by expansion of blast furnace slages have considerable utility as aggregates in the making of concrete, as components which are added to other minerals in the fabrication of cement, as fillers and the like for a wide variety of compositions and processes and for many other purposes.
There are generally three conventional processes for the making of expanded slags which are described, for example, at pages 247 ff of THE MAKING, SHAPING AND TREATING OF STEEL, U.S. Steel Company, 9th Edition, 1971, these processes yielding a porous product of low density.
In a first process, which can be described as free expansion, the slag passes along an inclined surface over a layer of water. This process has not been found to be practical except with easily expanded slags such as very hot high-silica or siliceous slags.
A second process, which can be described as semi-forced expansion, requires the use of mechanical means to increase the intensity of contact of gas, such as steam, with the slag and hence the development of expansion.
Finally, we can mention forced expansion where water vapor under pressure is forced through a layer of slag, for example, in a ladle or other receptacle having a bottom which is perforated or otherwise provided with means for injecting water or water vapor into the slag.
The last two processes have been found to be difficult to carry out and delicate from the point of view of the uniformity of the product produced. Even slight changes in the parameters of the expansion operation can result in wide variation in the porosity and homogeneity of the expanded slag.
There is known a semiforced expansion which involves the use of a vaned or finned drum to subdivide the slag and whereby a jet of slag is permitted to cascase onto the drum and water is directed against this jet by the drum.
The drum serves to subdivide the pyroplastic mass of the slag into small particles which travel through the air and are solidified by cooling therein. The homogeneity of the product obtained by this process is poor and attempts to use such disintegrating drums with or without bucket wheels or the like for subdividing the slag also proved to be ineffective.
In general, the prior art techniques for the expansion of slag have been found to be incapable of obtaining sufficient expansion, especially when the slag has a high degree of basicity or low temperature. Thus they have not been found to be practical with the slag of Thomas melts.